Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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Petrology, geochemistry and tectonic implications of volcanics dredged from the intersection of the Yap and Mariana trenches

Authors:	AJ Crawford L Beccaluva G Serri J Dostal

Institution:	¹Geology Department, University of Tasmania, G.P.O. Box 252C, Hobart, Tasmania 7001Australia ²Istituto di Petrologia, Universita di Napoli, Via Mezzacannone, 9, Napoli Italy ³Istituto di Petrologia, Universita di Pisa, Via S. Maria, 53, Pisa Italy ⁴Geology Department, St. Mary's University, Halifax, Nova Scotia Canada

Abstract:	Rocks dredged from the forearc very close to the intersection of the Yap and Mariana trenches include a suite of highly depleted arc tholeiites, and several samples of transitional to slightly alkaline basalt. The tholeiites range from magnesian quartz tholeiites with 0.46–0.6% TiO₂, to andesites with up to 62% SiO₂ and 8.2% FeO^*. All show pronounced LREE depletion and have very low contents of Ba and Sr. They are postulated to have been produced by partial melting of upper mantle peridotite residual after MORB extraction, following influx of hydrous fluids from the subducted slab. While these fluids were responsible for small enrichments in Ba, K, Rb and Sr in melts generated, LREE were not involved in the metasomatism, and the strong LREE depletion probably reflects the unmodified, depleted source peridotite. The second lava suite includes slightly Ne-normative, Ti-augite-bearing basalts with convex-upward REE patterns, showing slight LREE depletion ((La/Sm)_N = 0.76). The chemical features of these basalts support affinities with basalts erupted during the earliest stages of backarc basin opening. A KAr age on one sample(7.8 ± 1.3m.y.) is in good agreement with the initial opening of the Mariana Trough. The tectonic significance of the dredged arc tholeiite suite is less obvious. A KAr age of10.8 ± 0.4 My on one andesite, and the occurrence of similar lavas in dredges from at least 300 km along the length of the Yap arc, suggest that subduction was occurring beneath the Yap arc in the Late Miocene, after overthrusting of the Yap greenschist allochthon, and while calc-alkaline arc magmatism was occurring further north on the West Mariana Ridge. We suggest that the depleted arc tholeiites in dredge 1438 were generated by abnormally shallow melting of upper mantle beneath the Yap forearc following subduction beneath this area of young, hot Sorol Trough crust. These arc tholeiites represent a magma type transitional between more typical arc tholeiites (e.g. Tongan) and high-Mg andesites and boninites.

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